In his clear and entertaining way, Master Edgar explains the ins and outs of varnish. A couple of personal notes:
I’ve used both spirit and oil, and trust me, spirit varnishing is VERY TRICKY! It requires technique, technique, technique.
Edgar mentions ‘craquelure’ in his video. Unless you’re into old artwork and paintings, it’s not a word with which you would likely be familiar. I’ve learned that it can be a thing of beauty in vintage violins… a badge of honour, as it were.
Craquelure (French: craquelé; Italian: crettatura) is a fine pattern of dense cracking formed on the surface of materials. It can be a result of drying, aging, intentional patterning, or a combination of all three. Recently, analysis of craquelure has been proposed as a way to authenticate art.
The Prairie Fiddler 